The present invention relates to an apparatus for simultaneously reading from or writing to different information carrier layers of an optical recording medium having at least two different information carrier layers.
Recording media of this type are used for increasing the data capacity. In this case, the information carrier layers are semi-transparent and are each arranged parallel to one another. Their spatial separation is typically from 30 to 50 xcexcm with a tolerance of xc2x15 xcexcm. Crosstalk during the read-out of the data is suppressed on account of the spatial separation. For numerous applications, it may be expedient to read out data simultaneously from two or more information carrier layers, or to read out data and simultaneously write in other data. Simultaneously reading from or writing to two or more information carrier layers requires light to be focused correctly onto the generally spirally or circularly arranged data tracks of the various information carrier layers and the corresponding focal points to be tracked in the tracking direction, that is to say generally perpendicularly to the direction of the track, in the plane of the tracks. Since, on account of manufacturing tolerances, the information carrier layers are neither arranged exactly equidistantly over the entire plane of the recording medium, and nor do the data tracks lie congruently one above the other, it is necessary to track the two focal points both axially, that is to say in the direction of focus, and laterally, that is to say in the tracking direction i.e. perpendicular to the track. Simultaneous reading or writing likewise enables the data rate to be increased. One possible way of reading two or more layers in parallel consists in using two or more scanners. However, this is complicated and expensive.
EP-A-0 837 455 discloses an apparatus for simultaneously reading from or writing to different information carriers of an optical recording medium having at least two different information carrier layers. This apparatus manages with a single scanner. It has a partial beam generating means for generating different partial beams, a focusing means for focusing the partial beams onto different information carrier layers and a detection means with at least one detector element for each partial beam. The known apparatus may be regarded as having the disadvantage that the partial beams traverse a largely common optical path, as a result of which only slight tolerances with regard to the spacing of the information carrier layers from one another and the relative deviation of information tracks of the different information carrier layers laterally with respect to one another are tolerated.
An object of the present invention is to propose an apparatus which provides a largely common optical path for the individual partial beams and is nevertheless more tolerant with regard to the aforementioned deviations.
To that end, according to the invention, at least one partial beam generating means is provided for coupling a partial beam into and out of a pencil of rays and also a beam influencing means is provided for independently influencing the coupled-out partial beam. This has the advantage that a largely common optical path can be used for the concentrated partial beams from their generation through to the detection, as a result of which a small number of components, a compact structure of the scanner and lower production costs are made possible. Nevertheless, it is possible to correct tolerances with regard to the spacing of the individual information carrier layers from one another and individual information tracks of different information carrier layers with respect to one another, the read-out and writing reliability thereby being increased. That means that track following or focussing or both is, according to the invention, performed individually for at least two different partial beams. It likewise lies within the scope of the invention to use a partial beam generating means only for coupling in a partial beam or only for coupling out a partial beam.
A focusing means for focus and track regulation is advantageously provided as beam influencing means for each partial beam. This has the advantage of being cost-effective since it is possible to use conventional focusing means the number of which is merely increased. A further advantage resides in the fact that it is possible to compensate for large tolerances since the focusing means enable relatively large amounts of travel both in the axial direction and in the lateral direction. In this case, it is likewise possible simultaneously to scan on a single information carrier layer. In this context, scanning means not only reading, writing, following without reading or writing but also reading and writing an information track of an information carrier layer.
The partial beam generating means is advantageously a polarizing beam splitter. This means that only a single component is necessary for this. A further advantage resides in the fact that a different intensity of the partial beams can be set by rotating the plane of polarization of the incident beam, for example by means of rotating a laser serving as the light source. This is particularly advantageous when the intention is to use one partial beam to effect reading but the other to effect writing, since writing generally necessitates a higher power. However, the variability of the intensities is also advantageous for other applications, for example for reading from an information layer which is situated at a deeper level with respect to the entry plane of the light into the recording medium by means of a higher intensity in comparison with reading from an information carrier layer which is situated at a higher level. Compensation of the greater attenuation of the corresponding partial beam due to the larger number of information carrier layers to be traversed is obtained as a result of this.
The invention provides a divider, at one input of which a detector output signal associated with one of the partial beams is present and at the other input of which a signal corresponding to the control signal for the power of the light source is present. This has the advantage that, by virtue of the division of detector output signal by light source power, even in the event of modulation of the light source power, a data signal which is not interfered with by this modulation is generated. Modulation of this type is employed particularly in the case of writing to an information carrier layer, as a result of which, however, crosstalk to the other partial beam, that is to say modulation of this partial beam, is effected if the latter originates from the same light source. Crosstalk of this type is avoided according to the invention.
A further variant of the invention provides for a single focusing means but different beam influencing means to be provided for a plurality of partial beams. This has the advantage that only a single focusing means is necessary, which focuses all the partial beams onto their respective information carrier layer and uniformly tracks the corresponding information tracks. A beam influencing means is provided in each case for the second and each further partial beam and influences these independently of the first partial beam. The first partial beam is provided without separate beam influencing means but may likewise have one if required. Variations in the spacing of the information carrier layers from one another and the information tracks in the lateral direction with respect to one another are carried out separately for each of the partial beams, while the coarse tracking is effected by way of the common focusing means.
The invention provides for the partial beam generating means to have a dedicated light source for each partial beam. This has the advantage that it is possible to simultaneously write different contents on a plurality of information carrier layers without crosstalk occurring, since the power of the light sources can be regulated individually. This measure likewise has the advantage that reading can be effected with different power.
The beam influencing means is advantageously a collimator lens. The collimator lens, which is present in any case, is thus also used as beam influencing means with only a small additional outlay. For this purpose, it is arranged such that it can move in the axial and/or lateral direction and is provided with corresponding actuators. The latter may be electromagnetically actuated actuators of the kind known for driving the focusing means, piezoelectric, magnetostrictive or other suitable actuators.
As an alternative to this or in addition, it is provided that the beam influencing means has an optically transparent element which is tuneable in terms of the refractive index. In the simplest case, the said element is a glass plate which is introduced into the beam path or is removed therefrom, a plurality of such laminae having a different thickness or a different refractive index, or other suitable elements. This has the advantage that tracking of the focusing is made possible without a high outlay, if appropriate in interaction with further beam influencing means, such as a moveable collimator lens.
According to the invention, light source and associated detector are arranged in an optically conjugate position with respect to one another. This has the advantage that the light spot falling onto the respective detector is not displaced, or is only slightly displaced, by actuation of the beam influencing means.
Further advantages of the invention are specified in the following description of advantageous configurations with reference to the figures. It is understood that the features specified can also expediently be combined and/or modified without departing from the scope of the present invention.